1. Field of the Invention
This invention relates to the inspection of silicon wafers as used in semiconductor manufacturing processes and, in particular, to a system for automatically performing inspections of a patterned wafer.
2. Description of the Prior Art
Although the inspection of unpatterned wafers for cleanliness has alone been automated, patterned wafers continue to be inspected manually. In this case, a patterned wafer is placed on a wafer chuck under a bright light and is positioned to avoid reflections so as not to blind the viewer. At the same time, the position of the light with respect to the angle of the wafer must be such as to pick up scattered light from particulate contamination. Also, large defects may be detected. The full wafer is inspected and some of the characteristics of the present day manual macro inspection are: a quick check of large defects specifically particulate contamination down to approximately ten microns; checks for non-uniformities of exposure or development, and checks for poor spin; either backsplash, starburst, penumbral effect, contamination, etc.
Typically, micro inspections today are done by an inspection operator using a standard microscope. The areas of inspection include (1) registration checking, (2) pattern check for systematic defects, and (3) point defect checking. The latter can be typified in one of two ways: either (3a) random defects, or (3b) repeating defects. Random defects are common to all photo processes. The resulting defect density is recognized as a major yield limiter and as such must be monitored to provide data for yield control. Typical sources of random defects are random mass defects, defects in films (including resist), air born and liquid born particulate contamination. Repeating defects from a mask or reticle are a major cause of yield loss; in particular when using stepping lithography, the repeating defect prints everytime the reticle field is printed.
Although manual inspection is rather simple, and requires relatively low cost equipment, the results are somewhat inconsistent because of the subjective nature of the assessment and the attention span of the operator. Further, the time required to process the wafer as well as the limited amount of information that may be readily obtained limits the application of the manual inspection techniques to statistical sampling.